This invention relates to a method of calibrating a spectrophotometer provided with a monochromator for outputting a monochromatic beam of light with a specified wavelength such that the difference between the wavelength of the actually outputted light and the specified wavelength can be eliminated.
Spectrophotometers such as ultraviolet, visible light and atomic absorption spectrophotometers are provided with a monochromator for obtaining a monochromatic beam of light with a specified wavelength. In general, a monochromator comprises a wavelength-dispersing element such as a diffraction grating or a prism and a rotary mechanism for changing the orientation of such a wavelength-dispersing element with respect to an incident beam of light. The orientation of the wavelength-dispersing element is adjusted by the rotary mechanism such that a monochromatic beam of light with a specified wavelength can be outputted through a fixed slit. Since a diffraction grating is widely being used as the wavelength-dispersing element, the wavelength-dispersion element will be referred to as the diffraction grating in the following description.
As for the rotary mechanism for changing the orientation of the diffraction grating of a monochromator, it has been known to make use of a sine bar mechanism to convert a linear motion into a rotary motion, an open-loop control by means of a combination of a stepping motor and a deceleration gear mechanism or a closed-loop control by means of a DC servo motor. If a combination of a stepping motor and a deceleration gear mechanism is used, for example, the known formula of optics relating the orientation of the diffraction grating with the wavelength of the diffracted output light, as well as the design relationship between the angular position of the drive shaft of the stepping motor and the rotary angle of the deceleration gear mechanism, must be considered to establish an ideal relationship between the number of driving pulses which controls the motor and the wavelength of the output light.
Because of many factors such as the mechanical errors in precision by the rotary mechanism, errors in the diffraction constant of the diffraction grating or in the coefficient of refraction of the prism, and errors in the positioning of the optical components, however, the wavelength of the output light is not always equal to the target wavelength as accurately as desired. In the past, attempts have been made to reduce the inaccuracy by using more accurate mechanical and optical components and/or increasing the size of the monochromator itself such that the effect of the inaccuracy will be somewhat reduced. Such a method of improving the accuracy in wavelength is not desirable because of the cost and also because the spectrophotometer cannot be made compact.
More recently, there has been proposed a method whereby a spectrum of known wavelengths is used such as the zero-order bright line spectrum of a deuterium lamp and a spectrum of a mercury lamp with a plurality of bright lines. A variation which actually appears is obtained corresponding to a specified input to the rotary mechanism such as the number of driving pulses sent to a stepping motor and the monochromator is calibrated by using the observed variation.
A deuterium lamp and a mercury lamp emit at most only about ten bright lines and the intervals between these bright lines are not even. Moreover, the transmission error by a rotary mechanism does not increase of decrease uniformly with respect to the input angle. As a result, there is no guarantee that the calibration effort by such a prior art method is producing a desirable effect. The accuracy may be becoming worse by such an attempt at calibration.
It is therefore an object of this invention, in view of the above, to provide a method of calibrating a spectrophotometer accurately over the entire range of wavelength used thereby by using relatively inexpensive mechanical components and a light source which is easily obtainable.
A method according to this invention, with which the above and other objects can be accomplished, may be characterized as comprising the steps of preliminarily measuring transmission errors by the rotary mechanism for the monochromator to obtain an error curve with fluctuations for the rotary mechanism, selecting a smallest angular interval between a pair of feed angles corresponding to successive ones of peaks in the fluctuating error curve, selecting one or more light sources emitting bright lines with wavelength interval which corresponds to motion of the rotary mechanism by less than one half of the smallest angular interval, determining control values to be supplied to the rotary mechanism for obtaining monochromatic beams of light from the bright lines emitted from the light sources, producing from the control values and wavelength values of the bright lines a calibration table from which a required control value corresponding to a specified wavelength value can be retrieved, and storing this calibration table in a memory device.